Liquid crystal displays including backlights are widely used. In such a liquid crystal display, an image displayed on a liquid crystal display element is visualized by light that is emitted from a surface light source device placed in a backlight section and that is transmitted through the liquid crystal display element. Such surface light source devices for backlights are classified into two types; edge-light type surface light source devices and direct surface light source devices. Each edge-light type surface light source device has primary light sources in side surfaces of light guiding plates, and is more effective in thickness reduction than the direct surface light source devices. Accordingly, the edge-light type surface light source devices are widely used as display units of portable laptop computers, monitors and the like.
In such an edge-light type surface light source device, the light guiding plate is a panel of transparent resin or the like. This light guiding plate has an entrance end surface between an exit surface and a bottom surface opposite to the exit surface. In the entrance end surface, a primary light source such as a linear or dot-shaped light source is provided. Moreover, in the exit and bottom surfaces, white dots or fine patterns are formed for adjustment to increase efficiency in extracting light through the exit surface. Light emitted from the primary light source is therefore entered the light guiding plate through the entrance end surface, guided within the light guiding plate, and then emitted from the exit surface to a liquid crystal display element section.
As the light control pattern of the light guide, a parallel pattern formed in both of the exit and rear surfaces is known (for example, see Patent document 1).
In the light guide having such a shape, as shown in FIG. 1, there are extremely strong dark lines 11 seen in corners and notches of a light guide 3. In order to eliminate such a defect, it is necessary to use a diffusion sheet or the like for an improvement in the screen image quality while sacrificing the brightness performance.
For another purpose, there is a known method of adding a light-diffusive resin composition (a diffusing agent) for use in a light guide, in which an examination is being made for a wedge-type light guide to balance light transmission and diffusion by blending, at a low concentration, a diffusing agent being significantly different in refractive index from a base material and having a high diffusion efficiency (for example, see Patent document 2).
However, in the case of using the diffusing agent described in Patent document 2 as a method to reduce dark lines, the diffusing performance greatly changes as the concentration changes a little because of the high diffusion efficiency thereof. Accordingly, the aforementioned method is improper for a case where the diffusing performance needs to be finely changed like a case of reducing dark lines while keeping the optical performance.
Patent document 1: JP-A 2005-142164
Patent document 2: JP-A H7-199116